This project utilizes stereological and other morphometric techniques to measure membrane areas and organelle volume fractions in rat and rabbit myocardial cells during embryonic and post-natal growth and development. Changes in the volume of sarcoplasmic reticulum in association with changes in extracellular and intracellular ionic composition are also measured. The freeze fracture technique is used to look at changes in membrane particle distribution which occur during development or in association with changes in cellular ionic composition. Exchange of 36Cl with cardiac cellular Cl is measured both in vivo and in vitro in rat hearts in order to quantify the subcellular distribution of chloride. A morphometric analysis is also being done on the time course of changes in ultrastructure of left ventricular myocardial cells in the spontaneously hypertensive rat with and without preventive or therapeutic antihypertensive therapy. BIBLIOGRAPHIC REFERENCES: Smith, H. E., and E. Page, Morphometry of rat heart mitochondrial subcompartments and membranes: Application to myocardial cell atrophy after hypophysectomy, J Ultrastruct Res, 55, 31-41, 1976. Smith, H.E., and E. Page, Changes in cardiac mitochondrial ultrastructure during the embryonic-neonatal transition: A stereological study, Anat Rec, 184, 534, 1976 (abstract).